Libfmt is a port of Plan 9's formatted print library.
As a base it provides all the syntax of ANSI printf
but adds the ability for client programs to install
new print verbs.  One such print verb (installed by
default) is %r, which prints the system error string.
Instead of perror("foo"), you can write fprint(2, "foo: %r\n"). 
This is especially nice when you write verbs to format
the data structures used by your particular program.
